This invention relates to packaging films and, in particular, to a nonoriented packaging film. The packaging film is formulated to have a level of stiffness, memory and ease of tearing which is desirable for applications such as food packaging and tampon overwrap.
This invention concerns packaging films and, in particular, packaging films that can be easily opened by tearing but that, at the same time, protect the product contained within the film from dirt, dust, moisture or other contamination. Exemplary uses for such films include, for example, candy wrappers, tampon overwrap, ice cream wrappers, and individual snack food wrappers. It should, however, be understood that the film of the present invention may have many other uses. The packaging film has an optimized modulus level which allows the film to maintain its shape. For example, in the instance of use of the film as a wrapper for hard candy, the film is wrapped around the piece of hard candy and the protruding edges are twisted. Memory and secant modulus are attributes required to enable the film to keep its position and prevent the hard candy from becoming dislodged until forced out of film by an external force.
In response to this demand, biaxially oriented polypropylene (BOPP) films were produced and achieved an important position in the packaging industry, especially in the packaging of foods, owing to their stiffness, strength, clarity, and moisture barrier properties. Currently, this market segment accounts for about 70% of the BOPP films produced. In general, the films are generally multilayer structures produced by the so-called tenter process. In this process, the polymers are melted and homogenized in extruders. The melts are filtered, and brought together in a slot or annular die, where they are extruded to produce a monolayer or multilayer melt film. In slot die extrusion, the melt curtain is cast onto a chill roll, where it crystallizes. In annular die extrusion, the film is quenched with air to form a solid film. The film is subsequently oriented or stretched in the longitudinal and transverse directions. The stretching process is typically a three-stage process, as described, for example, in EP-0 116 457 B1. After being stretched, the film is cooled to room temperature, and trimmed at both edges. If desired, the film is then surface treated. The film is subsequently wound and packaged according to customer specifications. The application-relevant properties of BOPP films are principally determined by the biaxial orientation and the polymer compositions thereof. However, the biaxial orientation production process has many disadvantages. First, the process involves the use of expensive equipment and introduces an additional step into the manufacturing process, thereby resulting in an inefficient and costly production process. Second, due to the stress during processing of the film, the film is highly susceptible to breakdowns in the course of production, for example, tears in the film.
Several prior art films have been developed to replace BOPP films. One such prior art film is manufactured by coating a polymer film with a resin layer including a propylene copolymer, an xcex1-olefin having four or more carbons, and ethylene. The resulting film exhibits inferior tear properties, due to the presence of ethylene and the crystallinity of the polypropylene components. Further, the film necessitates lamination of the resin layer to the polymer film, which further decreases the tearability of the film.
Another prior art BOPP replacement film is manufactured by orienting the film in only one direction. However, this process involves the use of outdated film orientation equipment, and although it eliminates one processing step, the remaining orientation step is still time-consuming, expensive, and increases the frequency of processing breakdown.
An additional prior art BOPP replacement film involves the addition of a weakening line in the film or scoring the film itself. Alternatively, a row of notches may be added at each end seal of the wrap. However, these packaging films are relatively expensive to produce because of the additional equipment costs and the additional manufacturing steps. Further, where the tear lines are beyond the enclosed product, a great deal of wrap material is wasted, thereby increasing processing costs.
Yet another prior art packaging film involves the addition of a tearstrip. The tearstrip is a narrow band of supportive material adhered along the inside periphery of the wrapped product packaging and ending in a tab that is notched on either side. As the tab is lifted and pulled, due to the notching and pull force, a tear begins to propagate in the direction of the pull. However, the addition of the tab is a costly alternative.
It would be desirable, therefore, to provide a cost-effective packaging film with good stiffness, low tensile elongation at yield, good dead-fold characteristics, and good tearability.
An object of the present invention is to provide a packaging film that does not require orienting to have good tearability and stiffness.
A further object of the present invention is to provide a waterproof packaging film with increased clarity and adequate dead-fold characteristics.
The present invention is a nonoriented packaging film that includes standard polypropylene and highly crystalline polypropylene (hcPP) with a melt flow rate of 20 g/10 minutes or greater. HcPP is conventionally used in applications such as injection molding, as described in U.S. Pat. No. 6,162,887. U.S. Pat. No. 6,162,887 is incorporated herein by reference. The hcPP can be utilized to provide film with increased stiffness and crystallinity. While all polypropylene films develop some crystallinity as they age, it is the inherent, high level of crystallinity of hcPP that allows the inventive film to tear easily. Nucleating agents may be added to the film composition to attain desired dead-fold or memory and increased stiffness. Since no secondary or subsequent orientation processing step is required, the packaging film of the present invention is more cost-effective than prior art packaging films.
The film may be a single layer film or a multilayer film. In multilayer embodiment, skin layers with standard homopolymer polypropylene and no hcPP may encapsulate a core layer that includes hcPP and standard homopolymer polypropylene resins. Because the more crystalline core layer is positioned between two less crystalline layers, the film is more easily processed and has increased durability. Another multilayer embodiment has a core layer and two skin layer, each of the three layers containing hcPP resin.
The amount of hcPP resin contained within each layer may vary according to the properties desired in the film. For example, all layers may contain the same amount of hcPP resin. As a result, the layers have similar or the same viscosities and may be simultaneously co-extruded on standard co-extrusion equipment without the addition of processing acids.
Additional objects and advantages of this invention will become apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.